1. Field of the Invention
The present invention relates to an aluminum nitride sintered body, and more particularly to an aluminum nitride sintered body having high thermal conductivity which is produced from a starting powder prepared by direct nitriding of metallic aluminum, and a method of producing the same. The present invention also concerns an aluminum nitride green sheet produced from such a starting powder and a method for making it.
2. Prior Art
With the progress of large-scale integration in recent years, there is an increasing demand for insulated substrates having high thermal conductivity in order to package semiconductor elements generating large amounts of heat such as high integrated circuits, power transistors, laser diodes, and the like.
Sintered bodies based on beryllium oxide or beryllia (BeO) have conventionally been used as ceramic materials having high thermal conductivity, but the use of these materials in a wide variety of applications is limited due to their toxicity. As a substitute for beryllia, aluminum nitride (AlN) has been used in the production of highly thermal conductive substrates because it is stable and has high thermal conductivity in addition to good high-temperature strength and electric insulating properties.
As mentioned above, aluminum nitride possesses properties which make it suitable for use in the production of semiconductor substrates, and its theoretical thermal conductivity is estimated to be as high as about 300 W/mK. However, sintered bodies of aluminum nitride presently available on the market have generally much lower values of thermal conductivity on the order of 100 W/mK or less. Accordingly, improvement in thermal conductivity of aluminum nitride sintered bodies is still desired.
In one method of producing ceramic substrates in which a printing technique is used to form a desired conductive pattern such as a thick-film integrated circuit on the substrate, the conductive pattern is initially formed by means of screen printing on a green ceramic sheet, the printed green sheet is then punched to make blanks, and the blanks are piled one atop another, and are baked to form a ceramic IC package. This method can be performed inexpensively and is suitable for the mass-production of ceramic packages, particularly multilayered ceramic substrates or packages. In order to make aluminum nitride multilayered substrates using this method, it is desired to provide quality green sheets of aluminum nitride which have improved green strength and capable of producing high-quality sintered bodies, and a method of making such green sheets. Production of aluminum nitride green sheets from a starting powder prepared by the alumina reduction method mentioned below is disclosed in Japanese Patent Laid-Open Applications Nos. 60-171270 and 60-180964.
Since it is difficult to sinter aluminum nitride powder alone, it is conventionally sintered with the aid of a sintering additive, which is mixed with an aluminum nitride starting powder prior to shaping. Sintering additives which are known to be suitable for this purpose include compounds of Group IIa metals (alkaline earth metals), Group IIIa metals (rare earth metals), and Group IIIb metals (aluminum group metals) of the periodic table such as Y.sub.2 O.sub.3 (yttrium oxide or yttria), CaO (lime or calcium oxide), and CaC.sub.2 (calcium carbide). See Japanese Patent Laid-Open Applications Nos. 59-207814, 60-60910, 60-65768, 60-71575, etc.
There are two well-known methods for preparing aluminum nitride powder. One is the direct nitriding method in which metallic aluminum powder is directly nitrided with nitrogen or ammonia gas. The other is the alumina reduction method in which alumina powder is mixed with carbon and baked in nitrogen or ammonia gas to effect reduction of alumina and nitriding simultaneously.
In the direct nitriding method, the aluminum nitride powder product is usually contaminated with cationic impurities in an amount of at least a few percent by weight which enter the product from the grinding vessel or grinding media in the step of grinding the starting metallic aluminum material in order to increase the efficiency of nitriding or in the step of pulverizing the aluminum nitride powder formed by nitriding in order to reduce the particle size to one suitable for use in shaping and sintering. The pulverization of formed aluminum nitride powder is usually required since a considerable portion of the powder is agglomerated after nitriding. Also in the pulverization of the aluminum nitride powder, the surface of the powder is oxidized to a certain degree, and therefore the aluminum nitride powder product obtained by the direct nitriding method usually contains oxygen in an amount of at least 2% by weight, and in most cases at least 3% by weight. Such aluminum nitride powder containing oxygen and cationic impurities in such relatively large amounts are not suitable for use as starting materials to produce high-quality aluminum nitride sintered bodies. For this reason, the use of aluminum nitride powder obtained by the direct nitriding method in the production of sintered bodies having high thermal conductivity with the aid of a sintering additive is limited.
In the alumina reduction method, since agglomeration of particles does not occur significantly during nitriding, the starting alumina can be previously pulverized to the desired particle size prior to reduction an nitriding, and the resulting aluminum nitride powder can be used without further pulverization. Thus, according to this method, aluminum nitride powder having an average particle diameter of 2 .mu.m or less can be obtained and it can be directly used as a starting powder in the production of sintered bodies. Because of elimination of the pulverzation procedure after nitriding, aluminum nitride powder prepared by the alumina reduction method is relatively pure. Its content of cationic impurities can be readily decreased to 0.5% by weight or less, and its oxygen content is usually at most 3% by weight. In view of these advantages, in the prior art production of aluminum nitride sintered bodies, aluminum nitride powder obtained by the alumina reduction method has been used in most cases as a starting powder to be sintered with the aid of a sintering additive.
However, as mentioned above, most of the aluminum nitride sintered bodies obtained in the prior art from the above-mentioned relatively pure starting powder prepared by the alumina reduction method exhibit relatively low values of thermal conductivity on the order of 100 W/mK or lower, which is much lower than the theoretical value of about 300 W/mK.
Japanese Patent Laid-Open Applications Nos. 60-178688 and 61-91068 describe the production of aluminum nitride sintered bodies having a thermal conductivity of more than 100 W/mK, but neither of these teach the use of an aluminum nitride starting powder prepared by the direct nitriding method. In addition, in the method described in Japanese Patent Laid-Open Application No. 61-91068, a mixture of an aluminum nitride starting powder and a sintering additive is baked in the presence of free carbon or a carbonaceous substance as a deoxidizer in order to lower the oxygen content of the sintered product and thereby improve its thermal conductivity. This makes the overall operation very complicated.
With respect to the manufacturing process, the direct nitriding method can be performed by a very simple process of heating metallic aluminum powder in nitrogen or ammonia gas followed by pulverization. On the contrary, the alumina reduction method involves (1) thorough mixing of alumina powder and carbon powder, (2) heating of the mixed powder in nitrogen or ammonia gas, and (3) removal of the residual carbon by oxidation or burning. Because of its simplicity, the direct nitriding method can provide aluminum nitride powder at much lower cost than the alumina reduction method. Typically, the cost of manufacturing aluminum nitride powder by the direct nitriding method is from one-fourth to one-sixth of that by the alumina reduction method.
Accordingly, it is very advantageous to use an aluminum nitride starting powder prepared by the direct nitriding method in the production of aluminum nitride sintered bodies in order to decrease the manufacturing cost thereof. However, as mentioned above, the use of aluminum nitride powder prepared by the direct nitriding method has been limited, particularly in the production of sintered bodies having high thermal conductivity, due to its relatively low purity.